1. Field of the Invention
The present invention relates to an angle detection device such as an encoder, and more particularly to a device for detecting a rotation angle of a rotary reflection mirror for scanning a light beam from a light source such as a laser on a screen surface by a galvano scanner having the rotary reflection mirror.
2. Related Background Art
In a device for reflecting a light beam from a light source such as a laser beam by a rotary reflection mirror or a so-called galvano mirror and scanning a screen surface by the light beam, an encoder which detects a rotation angle of the rotary reflection mirror to control the drive of the rotary reflection mirror is usually used. It is disclosed, for example, in U.S. Pat. No. 4,829,342 and U.S. Pat. No. 4,868,385.
FIG. 6 shows a schematic view of such a prior art device. Numeral 1 denotes a rotary reflection mirror which is coupled to a driver 3 such as a motor by a rotary shaft 2. Numeral 4 denotes a display screen and numeral 5 denotes a light source such as a laser.
A light beam K from the laser 5 is directed to the display screen 4 through the rotary reflection mirror 1. The rotary reflection mirror 1 is rotated by the driver 3 so that the reflected light beam from the rotary reflection mirror 1 scans the display screen 4.
An angle sensor (not shown) for detecting a rotation angle of the rotary reflection mirror 1 (rotary shaft 2) is provided in the driver 3 so that the angle sensor produces an angular signal which is proportional to the rotation angle of the rotary reflection mirror 1.
The angular signal together with an input signal through a coordinate correction computer are fed back to a drive signal of the driver 3 to control the drive of the rotation of the rotary shaft 2.
However, the device shown in FIG. 6 has the following problem. FIG. 7 shows a partial plan view of FIG. 6. It shows the laser beam K and the rotary reflection mirror 1, and an incident position of the laser beam K on the display screen 4.
If the rotary reflection mirror 1 is rotated by an angle .theta., the laser beam K is deflected by the rotary reflection mirror 1 by an angle 2.theta.. The incident position of the laser beam K on the display screen 4, that is, a depiction point Y.sub.1 is given by EQU Y.sub.1 =L.multidot.tan 2.theta. (1)
where L is a distance from a center of rotation of the rotary reflection mirror 1 to the display screen 4.
Similarly, if the rotary reflection mirror 1 is rotated by an angle 2.theta., the depiction point on the display screen 4 is given by EQU Y.sub.2 =L.multidot.tan 4.theta. (2).
Accordingly, a distance .DELTA.Y between the depiction points Y.sub.1 and Y.sub.2 is given by ##EQU1##
Thus the depiction positions Y.sub.1 and Y.sub.2 are not equispaced with respect to the rotation angle.
As a result, as shown in FIG. 8, even if the rotary reflection mirror 1 is rotated by a constant angular pitch .theta., the depiction points Y on the display screen 4 are not equi-spaced.
A prior art angle detector used for the driver 3 usually produces a signal which is proportional to the rotation angle. Thus, there is no linear relation between the angular signal of the angle detector and the display screen 4.
Accordingly, in the prior art device, it is necessary to compensate for the linearity of the depiction points by the coordinate correction computer as shown in FIG. 6, and a signal processing circuit of the device is complex.